1. Field of the Invention
The present invention relates to an optical disk apparatus and a method of track access therefor, and more particularly to an improvement in the method of track access for an optical disk apparatus equipped with an optical pick-up provided with means for tracking a track formed on an optical disk.
2. Description of the Prior Art
Generally in the optical disk apparatus, an optical disk is maintained in rotation and a light beam is projected thereon by recording or reproducing means including an objective lens, etc. (hereinafter called optical pickup) to record or reproduce information. The optical pickup is generally provided with tracking means for control in such a manner that said light beam always exactly traces on tracks a track which is formed on the optical disk and in which information is recorded or is to be recorded. In such apparatus there is conducted an operation of transferring the irradiating position of the light beam from a track currently under tracking to another target track on which information recording or reproduction is desired. This operation is hereinafter called track access.
At the track access in the conventional optical disk apparatus, at first the distance to the target track is determined by detecting the current position of the optical pickup by means of an external scale or by reading track address information recorded in advance on the track, and the optical pickup is moved in the radial direction of the optical disk according to said distance.
However, the rotation of the optical disk usually involves an eccentricity corresponding to several tens to several hundreds of tracks due to an error in the precision of a center hole in the disk manufacture or an error in mounting of the disk on a turntable for disk rotation. Consequently, in such track access method, the reached track may be different from the target track because of the above-mentioned eccentricity. In such case it becomes necessary to repeat the track access operation or to execute a "kick" operation, to shift the light beam to a neighboring track by pulse driving the tracking means several times to several tens of times toward the target track, and there is accordingly required a long access time.
On the other hand, in the Japanese Patents Laid-open No. 62837/1983 and No. 91536/1983 there is disclosed a method of track access without the external scale, by directly detecting and calculating the number and direction of tracks crossed by the light beam at the displacement of the optical pickup. This method is, however, unable to achieve exact track access because errors often occur in said detection or calculation by dust or scars on the optical disk.